The present invention relates to a signal transmission system which performs DC power supply and signal transmission and reception through the use of a pair of signal lines.
In conventional systems of this kind in which signals are transmitted in two directions between first and second equipments via a pair of signal lines and a DC current is supplied from the first equipment to the second one, a constant-current circuit must be provided in the second equipment in order for the first equipment to make distinction between a load fluctuation current and a signal current of the second equipment. A fixed load current and a signal current flow in the pair of signal lines, and a change in the signal current is detected as a signal by a current detector of the first equipment; accordingly, the constant-current circuit is required to be capable of consuming power that a difference in the load fluctuation current of the second equipment is multiplied by the DC input voltage of the second equipment, and this leads to a considerable power loss, resulting in inefficient DC supply.